Which of the following actions would be categorized under supplier transactions?

The action of returning unsold items to a supplier directly pertains to supplier transactions, as it involves the relationship between a business and its suppliers. This transaction reflects a decrease in inventory and a financial adjustment related to purchases made from the supplier. It affects accounts payable and inventory levels, signifying the business’s obligation to return products not sold.

In contrast, canceling a customer invoice relates to customer transactions, as does creating a new customer account, which does not involve suppliers at all. Issuing a check to a customer also falls under customer transactions, which involve financial exchanges rather than supplier relationships. Therefore, returning unsold items to a supplier is the correct categorization within supplier transactions.